Metabolism of primaquine in normal human volunteers: investigation of phase I and phase II metabolites from plasma and urine using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry

Monoamine oxidase-A (MAO-A) low-expression variants and increased risk of Plasmodium vivax malaria relapses

OBJECTIVES

Primaquine is essential for the radical cure of Plasmodium vivax malaria and must be metabolized into its bioactive metabolites. Accordingly, polymorphisms in primaquine-metabolizing enzymes can impact the treatment efficacy. This pioneering study explores the influence of monoamine oxidase-A (MAO-A) on primaquine metabolism and its impact on malaria relapses.

METHODS

Samples from 205 patients with P. vivax malaria were retrospectively analysed by genotyping polymorphisms in MAO-A and cytochrome P450 2D6 (CYP2D6) genes. We measured the primaquine and carboxyprimaquine blood levels in 100 subjects for whom blood samples were available on the third day of treatment. We also examined the relationship between the enzyme variants and P. vivax malaria relapses in a group of subjects with well-documented relapses.

RESULTS

The median carboxyprimaquine level was significantly reduced in individuals carrying low-expression MAO-A alleles plus impaired CYP2D6. In addition, this group experienced significantly more P. vivax relapses. The low-expression MAO-A status was not associated with malaria relapses when CYP2D6 had normal activity. This suggests that the putative carboxyprimaquine contribution is irrelevant when the CYP2D6 pathway is fully active.

CONCLUSIONS

We found evidence that the low-expression MAO-A variants can potentiate the negative impact of impaired CYP2D6 activity, resulting in lower levels of carboxyprimaquine metabolite and multiple relapses. The findings support the hypothesis that carboxyprimaquine may be further metabolized through CYP-mediated pathways generating bioactive metabolites that act against the parasite.

Population pharmacokinetics of primaquine and its metabolites in African males

BACKGROUND

Primaquine (PQ) is the prototype 8-aminoquinoline drug, a class which targets gametocytes and hypnozoites. The World Health Organization (WHO) recommends adding a single low dose of primaquine to the standard artemisinin-based combination therapy (ACT) in order to block malaria transmission in regions with low malaria transmission. However, the haemolytic toxicity is a major adverse outcome of primaquine in glucose-6-phosphate dehydrogenase (G6PD)-deficient subjects. This study aimed to characterize the pharmacokinetic properties of primaquine and its major metabolites in G6PD-deficient subjects.

METHODS

A single low-dose of primaquine (0.4-0.5 mg/kg) was administered in twenty-eight African males. Venous and capillary plasma were sampled up to 24 h after the drug administration. Haemoglobin levels were observed up to 28 days after drug administration. Only PQ, carboxy-primaquine (CPQ), and primaquine carbamoyl-glucuronide (PQCG) were present in plasma samples and measured using liquid chromatography mass spectrometry. Drug and metabolites' pharmacokinetic properties were investigated using nonlinear mixed-effects modelling.

RESULTS

Population pharmacokinetic properties of PQ, CPQ, and PQCG can be described by one-compartment disposition kinetics with a transit-absorption model. Body weight was implemented as an allometric function on the clearance and volume parameters for all compounds. None of the covariates significantly affected the pharmacokinetic parameters. No significant correlations were detected between the exposures of the measured compounds and the change in haemoglobin or methaemoglobin levels. There was no significant haemoglobin drop in the G6PD-deficient patients after administration of a single low dose of PQ.

CONCLUSIONS

A single low-dose of PQ was haematologically safe in this population of G6PD-normal and G6PD-deficient African males without malaria. Trial registration NCT02535767.

An increase in urinary primaquine and a reduction in urinary primaquine-5,6-orthoquinone in the Thai population with CYP2D6 reduced enzyme function

Objectives

Primaquine is metabolized by the cytochrome P450-2D6 enzyme (CYP2D6) to an active primaquine-5,6-orthoquinone (POQ). No relationships of CYP2D6 polymorphisms with the pharmacokinetics of primaquine and POQ were reported in the Thai population.

Methods

We evaluated the genetic distribution of CYP2D6 in 345 Thai army populations together with the pharmacokinetic profiles of primaquine and POQ in plasma and urine (n = 44, descriptive data are presented in median (range)). All dose-related pharmacokinetic parameters were normalized by primaquine dose per body weight before statistical analysis.

Results

CYP2D6*10 was the allele observed with the highest frequency (56.62%) corresponding to CYP2D6*10/*10 (32.94%) and CYP2D6*1/*10 (27.94%) genotypes. CYP2D6 intermediate metabolizers (CYP2D6 IM) were found in 44.41% of the cohort and had an increase in the cumulative amount of primaquine excreted (CAE) in urine compared to normal metabolizers of CYP2D6 (CYP2D6 NM); (CYP2D6 IM vs. CYP2D6 NM: 2444 (1697-3564) vs. 1757 (1092-2185) μg/mg/kg, p = 0.039), a reduction in urine CAE of POQ (CYP2D6 IM vs CYP2D6 NM: 115 (46-297) vs. 318 (92-498) μg/mg/kg, p = 0.047) and a reduction in the POQ/primaquine CAE ratio in urine (CYP2D6 IM vs. CYP2D6 NM: 0.06 (0.01-0.11) vs. 0.16 (0.06-0.26), p = 0.009). No significant differences were found in the pharmacokinetic profiles of plasma primaquine and POQ.

Conclusions

The CYP2D6 polymorphisms influenced the changes in primaquine and POQ that were noticeable in the urine, supporting the role of the CYP2D6 gene testing before drug administration.

Pharmacogenomic variation in the Malagasy population: implications for the antimalarial drug primaquine metabolism

Aim: Antimalarial primaquine (PQ) eliminates liver hypnozoites of Plasmodium vivax. CYP2D6 gene variation contributes to PQ therapeutic failure. Additional gene variation may contribute to PQ efficacy. Information on pharmacogenomic variation in Madagascar, with vivax malaria and a unique population admixture, is scanty. Methods: The authors performed genome-wide genotyping of 55 Malagasy samples and analyzed data with a focus on a set of 28 pharmacogenes most relevant to PQ. Results: Mainly, the study identified 110 coding or splicing variants, including those that, based on previous studies in other populations, may be implicated in PQ response and copy number variation, specifically in chromosomal regions that contain pharmacogenes. Conclusion: With this pilot information, larger genome-wide association analyses with PQ metabolism and response are substantially more feasible.

Human Cytochrome P450 1, 2, 3 Families as Pharmacogenes with Emphases on Their Antimalarial and Antituberculosis Drugs and Prevalent African Alleles

Precision medicine gives individuals tailored medical treatment, with the genotype determining the therapeutic strategy, the appropriate dosage, and the likelihood of benefit or toxicity. Cytochrome P450 (CYP) enzyme families 1, 2, and 3 play a pivotal role in eliminating most drugs. Factors that affect CYP function and expression have a major impact on treatment outcomes. Therefore, polymorphisms of these enzymes result in alleles with diverse enzymatic activity and drug metabolism phenotypes. Africa has the highest CYP genetic diversity and also the highest burden of malaria and tuberculosis, and this review presents current general information on CYP enzymes together with variation data concerning antimalarial and antituberculosis drugs, while focusing on the first three CYP families. Afrocentric alleles such as CYP2A6*17, CYP2A6*23, CYP2A6*25, CYP2A6*28, CYP2B6*6, CYP2B6*18, CYP2C8*2, CYP2C9*5, CYP2C9*8, CYP2C9*9, CYP2C19*9, CYP2C19*13, CYP2C19*15, CYP2D6*2, CYP2D6*17, CYP2D6*29, and CYP3A4*15 are implicated in diverse metabolic phenotypes of different antimalarials such as artesunate, mefloquine, quinine, primaquine, and chloroquine. Moreover, CYP3A4, CYP1A1, CYP2C8, CYP2C18, CYP2C19, CYP2J2, and CYP1B1 are implicated in the metabolism of some second-line antituberculosis drugs such as bedaquiline and linezolid. Drug-drug interactions, induction/inhibition, and enzyme polymorphisms that influence the metabolism of antituberculosis, antimalarial, and other drugs, are explored. Moreover, a mapping of Afrocentric missense mutations to CYP structures and a documentation of their known effects provided structural insights, as understanding the mechanism of action of these enzymes and how the different alleles influence enzyme function is invaluable to the advancement of precision medicine.

Global perspectives on CYP2D6 associations with primaquine metabolism and Plasmodium vivax radical cure

Clinical trial and individual patient treatment outcomes have produced accumulating evidence that effective primaquine (PQ) treatment of Plasmodium vivax and P. ovale liver stage hypnozoites is associated with genetic variation in the human cytochrome P450 gene, CYP2D6. Successful PQ treatment of individual and population-wide infections by the Plasmodium species that generate these dormant liver stage forms is likely to be necessary to reach elimination of malaria caused by these parasites globally. Optimizing safe and effective PQ treatment will require coordination of efforts between the malaria and pharmacogenomics research communities.

Comparative single dose pharmacokinetics and metabolism of racemic primaquine and its enantiomers in human volunteers

Primaquine (PQ) is a racemic drug used in treatment of malaria for six decades. Recent studies suggest that the two enantiomers of PQ are differentially metabolized in animals, and this results in different pharmacological and toxicological profiles. The current study characterizes the pharmacokinetic (PK) properties, metabolism and tolerability of the individual enantiomers of PQ in healthy human volunteers with normal glucose-6-phosphate dehydrogenase (G6PD) activity. Two cohorts (at two dose levels), each with 18 subjects, participated in three study arms in a crossover fashion: a single dose of the (-)-R enantiomer (RPQ), a single dose of the (+)-S enantiomer (SPQ), and a single dose of racemic PQ (RSPQ). PQ and its key metabolites carboxyprimaquine (cPQ) and PQ-N-carbamoyl glucuronide (PQ-N-CG) were analyzed. Clear differences were observed in PK and metabolism of the two enantiomers. Relative PQ exposure was higher with SPQ as compared to RPQ. PQ maximum plasma concentration (C_max) and area under the plasma concentration-time curve were higher for SPQ, while the apparent volume of distribution and total body clearance were higher for RPQ. Metabolism of the two enantiomers showed dramatic differences: plasma PQ-N-CG was derived solely from SPQ, while RPQ was much more efficiently converted to cPQ than was SPQ. C_max of cPQ and PQ-N-CG were 10 and 2 times higher, respectively, than the parent drugs. The study demonstrates that the PK properties of PQ enantiomers show clear differences, and metabolism is highly enantioselective. Such differences in metabolism suggest potentially distinct toxicity profiles in multi-dose regimens, especially in G6PD-deficient subjects.

Comparative pharmacokinetics and tissue distribution of primaquine enantiomers in mice

Background

Primaquine (PQ) has been used for the radical cure of relapsing Plasmodium vivax malaria for more than 60 years. PQ is also recommended for prophylaxis and prevention of transmission of Plasmodium falciparum. However, clinical utility of PQ has been limited due to toxicity in individuals with genetic deficiencies in glucose 6-phosphate dehydrogenase (G6PD). PQ is currently approved for clinical use as a racemic mixture. Recent studies in animals as well as humans have established differential pharmacological and toxicological properties of the two enantiomers of PQ. This has been attributed to differential metabolism and pharmacokinetics of individual PQ enantiomers. The aim of the current study is to evaluate the comparative pharmacokinetics (PK), tissue distribution and metabolic profiles of the individual enantiomers in mice.

Methods

Two groups of 21 male Albino ND4 Swiss mice were dosed orally with 45 mg/kg of S-(+)-PQ and R-(−)PQ respectively. Each of the enantiomers was comprised of a 50:50 mixture of 12C- and 13C- stable isotope labelled species (at 6 carbons on the benzene ring of the quinoline core). Three mice were euthanized from each group at different time points (at 0, 0.5, 1, 2, 4, 8, 24 h) and blood was collected by terminal cardiac bleed. Liver, spleen, lungs, kidneys and brain were removed, extracted and analysed using UPLC/MS. The metabolites were profiled by tandem mass (MS/MS) fragmentation profile and fragments with 12C–13C twin peaks. Non-compartmental analysis was performed using the Phoenix WinNonLin PK software module.

Results

The plasma AUC0-last (µg h/mL) (1.6 vs. 0.6), T1/2 (h) (1.9 vs. 0.45), and Tmax (h) (1 vs. 0.5) were greater for SPQ as compared to RPQ. Generally, the concentration of SPQ was higher in all tissues. At Tmax, (0.5–1 h in all tissues), the level of SPQ was 3 times that of RPQ in the liver. Measured Cmax of SPQ and RPQ in the liver were about 100 and 40 times the Cmax values in plasma, respectively. Similar observations were recorded in other tissues where the concentration of SPQ was higher compared to RPQ (2× in the spleen, 6× in the kidneys, and 49× in the lungs) than in the plasma. CPQ, the major metabolite, was preferentially generated from RPQ, with higher levels in all tissues (> 10× in the liver, and 3.5× in the plasma) than from SPQ. The PQ-o-quinone was preferentially formed from the SPQ (> 4× compared to RPQ), with higher concentrations in the liver.

Conclusion

These studies show that in mice, PQ enantiomers are differentially biodistributed and metabolized, which may contribute to differential pharmacologic and toxicity profiles of PQ enantiomers. The findings on higher levels of PQ-o-quinone in liver and RBCs compared to plasma and preferential generation of this metabolite from SPQ are consistent with the higher anti-malarial efficacy of SPQ observed in the mouse causal prophylaxis test, and higher haemolytic toxicity in the humanized mouse model of G6PD deficiency. Potential relevance of these findings to clinical use of racemic PQ and other 8-aminoquinolines vis-à-vis need for further clinical evaluation of individual enantiomers are discussed.

Measurements of 5,6 orthoquinone, surrogate for presumed active primaquine metabolite 5-hydroxyprimaquine, in the urine of Cambodian adults

The active metabolites of primaquine, in particular 5-hydroxyprimaquine, likely responsible for clearance of dormant hypnozoites, are produced through the hepatic CYP450 2D6 (CYP2D6) enzymatic pathway. With the inherent instability of 5-hydroxyprimaquine, a stable surrogate, 5,6 orthoquinone, can now be detected and measured in the urine as part of primaquine pharmacokinetic studies. This study performed CYP450 2D6 genotyping and primaquine pharmacokinetic testing, to include urine 5,6 orthoquinone, in 27 healthy adult Cambodians, as a preliminary step to prepare for future clinical studies assessing primaquine efficacy for Plasmodium vivax infections. The CYP2D6 *10 reduced activity allele was found in 57% of volunteers, and the CYP2D6 genotypes were dominated by *1/*10 (33%) and *10/*10 (30%). Predicted phenotypes were evenly split between Normal Metabolizer (NM) and Intermediate Metabolizer (IM) except one volunteer with a gene duplication and unclear phenotype, classifying as either IM or NM. Median plasma PQ area under the curve (AUC) was lower in the NM group (460 hr*ng/mL) compared to the IM group (561 hr*ng/mL), although not statistically significant. Similar to what has been found in the US study, no 5,6 orthoquinone was detected in the plasma. The urine creatinine-corrected 5,6 orthoquinone AUC in the NM group was almost three times higher than in the IM group, with peak measurements (T max ) at 4 hours. Although there is variation among individuals, future studies examining the relationship between the levels of urine 5,6 orthoquinone and primaquine radical cure efficacy could result in a metabolism biomarker predictive of radical cure.

A Review of Genome-Based Precision Medicine Efforts Within the Department of Defense

INTRODUCTION

Providing patient-specific clinical care is an expanding focus for medical professionals and researchers, more commonly referred to as personalized or precision medicine. The goal of using a patient-centric approach is to provide safer care while also increasing the probability of therapeutic success through careful consideration of the influence of certain extrinsic and intrinsic human factors in developing the patient care plan. Of increasing influence on patient care is the phenotype and genotype information gathered from employing various next-generation sequencing methods. Guided by and partnered with our civilian colleagues, clinical components within the DoD are embracing and advancing genomic medicine in many facets-from the bench to the bedside-and in many therapeutic areas, from Psychiatry to Oncology. In this PubMed-based review, we describe published clinical research and interventions within the DoD using genome-informed data and emphasize precision medicine efforts in earlier stages of development with the potential to revolutionize the approach to therapeutics.

MATERIALS AND METHODS

The new PubMed database was searched for articles published between 2015 and 2020 with the following key search terms: precision medicine, genomic, pharmacogenetic, pharmacogenomic, US military, and Department of Defense.

RESULTS

Eighty-one articles were retrieved in our initial search. After screening the abstracts for studies that only involved direct testing of (or clinical interaction with) active duty, Reserve, National Guard, or civilian personnel working within the DoD and excluding any epidemiological or microbial isolation studies, seven were included in this review.

CONCLUSION

There are several programs and studies within the DoD, which investigate or use gene-based biomarkers or gene variants to deliver more precise clinical assessment and treatment. These genome-based precision medicine efforts aim to optimize the clinical care of DoD beneficiaries, particularly service members in the operational environment.

Mechanistic Modelling of Primaquine Pharmacokinetics, Gametocytocidal Activity, and Mosquito Infectivity

Clinical studies have shown that adding a single 0.25mg base/kg dose of primaquine to standard antimalarial regimens rapidly sterilises Plasmodium falciparum gametocytes. However, the mechanism of action and overall impact on malaria transmission is still unknown. Using data from 81 adult Malians with P. falciparum gametocytaemia who received the standard dihydroartemisinin-piperaquine treatment course and were randomised to receive either a single dose of primaquine between 0.0625 and 0.5 mg base/kg or placebo. We characterised the pharmacokinetic-pharmacodynamic relationships for transmission blocking activity. Both gametocyte clearance and mosquito infectivity were assessed. A mechanistically-linked pharmacokinetic-pharmacodynamic model adequately described primaquine and carboxy-primaquine pharmacokinetics, gametocyte dynamics, and mosquito infectivity at different clinical doses of primaquine. Primaquine showed a dose-dependent gametocytocidal effect that precedes clearance. A single low dose of primaquine (0.25 mg/kg) rapidly prevented P. falciparum transmissibility.

Determinants of primaquine and carboxyprimaquine exposures in children and adults with Plasmodium vivax malaria

Background Primaquine is the only widely available drug for radical cure of Plasmodium vivax malaria. There is uncertainty whether the pharmacokinetic properties of primaquine are altered significantly in childhood or not. Methods Glucose-6-phosphate dehydrogenase normal patients with uncomplicated P. vivax malaria were randomized to receive either chloroquine (25mg base/kg) or dihydroartemisinin-piperaquine (dihydroartemisinin 7mg/kg and piperaquine 55mg/kg) plus primaquine; given either as 0.5 mg base/kg/day for 14 days or 1 mg/kg/day for 7 days. Pre-dose day 7 venous plasma concentrations of chloroquine, desethylchloroquine, piperaquine, primaquine and carboxyprimaquine were measured. Methemoglobin levels were measured on day 7. Results Day 7 primaquine and carboxyprimaquine concentrations were available for 641 patients. After adjustment for the primaquine mg/kg daily dose, day of sampling, partner drug, and fever clearance, there was a significant non-linear relationship between age and trough primaquine and carboxyprimaquine concentrations, and day methemoglobin levels. Compared to adults 30 years of age, children 5 years of age had trough primaquine concentrations 0.53 (95% CI: 0.39- 0.73) fold lower, trough carboxyprimaquine concentrations 0.45 (95% CI: 0.35- 0.55) fold lower, and day 7 methemoglobin levels 0.87 (95% CI: 0.58-1.27) fold lower. Increasing concentrations of piperaquine and chloroquine and poor metabolizer CYP 2D6 alleles were associated with higher day 7 primaquine and carboxyprimaquine concentrations. Higher blood methemoglobin concentrations were associated with a lower risk of recurrence. Conclusion Young children have lower primaquine and carboxyprimaquine exposures, and lower levels of methemoglobinemia, than adults. Young children may need higher weight adjusted primaquine doses than adults.

Simplified and Rapid Determination of Primaquine and 5,6-Orthoquinone Primaquine by UHPLC-MS/MS: Its Application to a Pharmacokinetic Study

The method for the determination of primaquine (PQ) and 5,6-orthoquinone primaquine (5,6-PQ), the representative marker for PQ active metabolites, via CYP2D6 in human plasma and urine has been validated. All samples were extracted using acetonitrile for protein precipitation and analyzed using the ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) system. Chromatography separation was carried out using a Hypersil GOLD^TM aQ C₁₈ column (100 × 2.1 mm, particle size 1.9 μm) with a C₁₈ guard column (4 × 3 mm) flowed with an isocratic mode of methanol, water, and acetonitrile in an optimal ratio at 0.4 mL/min. The retention times of 5,6-PQ and PQ in plasma and urine were 0.8 and 1.6 min, respectively. The method was validated according to the guideline. The linearity of the analytes was in the range of 25–1500 ng/mL. The matrix effect of PQ and 5,6-PQ ranged from 100% to 116% and from 87% to 104% for plasma, and from 87% to 89% and from 86% to 87% for urine, respectively. The recovery of PQ and 5,6-PQ ranged from 78% to 95% and form 80% to 98% for plasma, and from 102% to from 112% to 97% to 109% for urine, respectively. The accuracy and precision of PQ and 5,6-PQ in plasma and urine were within the acceptance criteria. The samples should be kept in the freezer (−80 °C) and analyzed within 7 days due to the metabolite stability. This validated UHPLC-MS/MS method was beneficial for a pharmacokinetic study in subjects receiving PQ.

Mass spectrometry imaging in zebrafish larvae for assessing drug safety and metabolism

Drug safety assessment in the early phases of drug discovery is critical to facilitate the rapid development of novel therapeutics. Recently, teleost zebrafish (Danio rerio) has emerged as a promising vertebrate model for the assessment of drug safety. Zebrafish is a convenient model because of its small size, high fecundity, embryo transparency, and ex utero development. In this study, we developed a matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) method applied to zebrafish larvae to investigate safety and metabolism of sahaquine (Sq), an anticancer agent inhibiting histone deacetylase 6. This technique improves on prior studies using liquid chromatography-mass spectrometry (LC-MS) by adding analysis of the drug spatial distribution. Using this method, it was determined that Sq dissolved in fish water (1-2000 μM) did not reach the larval body and was mainly distributed throughout the yolk. High Sq concentration (800 μM) administered intravenously allowed the compound to reach the larval body but did not induce phenotypic abnormalities. Sq was metabolized into its glucuronidated form within 24 h and was excreted within 72 h. MALDI MSI was instrumental in showing that Sq-glucuronide was mainly formed in the gut and slightly in yolk syncytial layer, and provided valuable insights into xenobiotics elimination in zebrafish larvae. This study indicates that Sq has a good safety profile and merits further investigations in other disease models. In addition, the optimized MALDI MSI protocol provided here can be widely applied to study distribution and metabolic fate of other structurally related molecules.

Transplacental Transfer of Primaquine and Neurobehavioral Development of Prenatally Exposed Rats

Primaquine (PQ) not only eliminates P. falciparum gametocytes but also kills liver dormant forms of P. vivax and P. ovale. Owing to these unique therapeutic properties, it is an essential drug. Although PQ has been used for over 70 years, its toxicological database has gaps such as the absence of studies on its reproductive and developmental toxicity and kinetics in pregnancy. This study investigated the transplacental transfer of PQ and the effects of intrauterine exposure on the postnatal growth, survival, and neurobehavioral development of the offspring. PQ kinetics and transplacental transfer were investigated in rats treated orally (40 mg.kg·bw⁻¹) on gestation day (GD) 21. PQ was analyzed by high-performance liquid chromatography with diode array ultraviolet detection. To evaluate effects of intrauterine exposure on postnatal development, dams were treated orally with PQ (20 mg.kg·bw⁻¹·d⁻¹) or water (controls) on GD 0–21. Postnatal survival, body weight gain, somatic maturation, and reflex acquisition were evaluated. The open field test (OF) was conducted on PND 25. PQ concentration in the fetal plasma was nearly half that in maternal plasma. Except for increase in pregnancy loss, no effects of PQ were noted at term pregnancy and first days of life. Prenatal PQ did not affect postnatal weight gain nor did it impair somatic and neurologic development of the offspring. Pups born to PQ-treated dams showed reduced exploration and enhanced emotionality in the OF. PQ given in pregnancy, at doses greater than those recommended for malaria therapy, may affect pup postnatal survival and emotional behavior.

Enantioselective Interactions of Anti-Infective 8-Aminoquinoline Therapeutics with Human Monoamine Oxidases A and B

8-Aminoquinolines (8-AQs) are an important class of anti-infective therapeutics. The monoamine oxidases (MAOs) play a key role in metabolism of 8-AQs. A major role for MAO-A in metabolism of primaquine (PQ), the prototypical 8-AQ antimalarial, has been demonstrated. These investigations were further extended to characterize the enantioselective interactions of PQ and NPC1161 (8-[(4-amino-1-methylbutyl) amino]-5-[3, 4-dichlorophenoxy]-6-methoxy-4-methylquinoline) with human MAO-A and -B. NPC1161B, the (R)-(−) enantiomer with outstanding potential for malaria radical cure, treatment of visceral leishmaniasis and pneumocystis pneumonia infections is poised for clinical development. PQ showed moderate inhibition of human MAO-A and -B. Racemic PQ and (R)-(−)-PQ both showed marginally greater (1.2- and 1.6-fold, respectively) inhibition of MAO-A as compared to MAO-B. However, (S)-(+)-PQ showed a reverse selectivity with greater inhibition of MAO-B than MAO-A. Racemic NPC1161 was a strong inhibitor of MAOs with 3.7-fold selectivity against MAO-B compared to MAO-A. The (S)-(+) enantiomer (NPC1161A) was a better inhibitor of MAO-A and -B compared to the (R)-(−) enantiomer (NPC1161B), with more than 10-fold selectivity for inhibition of MAO-B over MAO-A. The enantioselective interaction of NPC1161 and strong binding of NPC1161A with MAO-B was further confirmed by enzyme-inhibitor binding and computational docking analyses. Differential interactions of PQ and NPC1161 enantiomers with human MAOs may contribute to the enantioselective pharmacodynamics and toxicity of anti-infective 8-AQs therapeutics.

CYP2D6 Genetic Variation and Its Implication for Vivax Malaria Treatment in Madagascar

Plasmodium vivax is one of the five human malaria parasite species, which has a wide geographical distribution and can cause severe disease and fatal outcomes. It has the ability to relapse from dormant liver stages (hypnozoites), weeks to months after clearance of the acute blood-stage infection. An 8-aminoquinoline drug primaquine (PQ) can clear the hypnozoites, and thus can be used as an anti-relapse therapeutic agent. Recently, a number of studies have found that its efficacy is compromised by polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene; decreased or absence of CYP2D6 activity contributes to PQ therapeutic failure. The present study sought to characterize CYP2D6 genetic variation in Madagascar, where populations originated from admixture between Asian and African populations, vivax malaria is endemic, and PQ can be deployed soon to achieve national malaria elimination. In a total of 211 samples collected from two health districts, CYP2D6 decreased function alleles CYP2D6*10, *17, *29, *36+*10, and *41 were observed at frequencies of 3.55-17.06%. In addition, nonfunctional alleles were observed, the most common of which were CYP2D6*4 (2.13%), *5 (1.66%), and the *4x2 gene duplication (1.42%). Given these frequencies, 34.6% of the individuals were predicted to be intermediate metabolizers (IM) with an enzyme activity score (AS) ≤ 1.0; both the IM phenotype and AS ≤ 1.0 have been found to be associated with PQ therapeutic failure. Furthermore, the allele and genotype frequency distributions add to the archaeological and genomic evidence of Malagasy populations constituting a unique, Asian-African admixed origin. The results from this exploratory study provide fresh insights about genomic characteristics that could affect the metabolism of PQ into its active state, and may enable optimization of PQ treatment across human genetic diversity, which is critical for achieving P. vivax elimination.

Monitoring Plasmodium vivax resistance to antimalarials: Persisting challenges and future directions

Emerging antimalarial drug resistance may undermine current efforts to control and eliminate Plasmodium vivax, the most geographically widespread yet neglected human malaria parasite. Endemic countries are expected to assess regularly the therapeutic efficacy of antimalarial drugs in use in order to adjust their malaria treatment policies, but proper funding and trained human resources are often lacking to execute relatively complex and expensive clinical studies, ideally complemented by ex vivo assays of drug resistance. Here we review the challenges for assessing in vivo P. vivax responses to commonly used antimalarials, especially chloroquine and primaquine, in the presence of confounding factors such as variable drug absorption, metabolism and interaction, and the risk of new infections following successful radical cure. We introduce a simple modeling approach to quantify the relative contribution of relapses and new infections to recurring parasitemias in clinical studies of hypnozoitocides. Finally, we examine recent methodological advances that may render ex vivo assays more practical and widely used to confirm P. vivax drug resistance phenotypes in endemic settings and review current approaches to the development of robust genetic markers for monitoring chloroquine resistance in P. vivax populations.

Synthesis, Biological Activity and In Silico Pharmacokinetic Prediction of a New 2-Thioxo-Imidazoldidin-4-One of Primaquine

The discovery of novel antiparasitic drugs for neglected tropical diseases (NTDs) constitutes a global urgency and requires a range of innovative strategies to ensure a sustainable pipeline of lead compounds. Thus far, primaquine (PQ) is the only transmission-blocking antimalarial that is clinically available, displaying marked activity against gametocytes of all causative species of human malaria (Plasmodium spp.). Chagas disease, caused by Trypanosoma cruzi, is another PQ-sensitive illness besides malaria. One of the major drawbacks of PQ is its metabolism into carboxyprimaquine (CPQ), which is less active than the parent drug. In this study, we developed different synthetic pathways to confer N-protection to PQ through introduction of thioxo-imidazolidin-4-one. The introduction of this group prevents the formation of CPQ, counteracting one major drawback of the parent drug. After that, we evaluated the potential biological activity of the novel 2-thioxo-imidazolidin-4-one derivative of PQ, which showed relevant in vitro activity against Trypanosoma cruzi (IC₅₀ 1.4 μM) compared to PQ (IC₅₀ 1.7 μM) and the reference drug benznidazole (IC₅₀ 1.6 μM). Noting its acceptable pharmacokinetic profile, this PQ conjugate may be a potential scaffold for novel drug exploration against Chagas disease.

Chloroquine Potentiates Primaquine Activity against Active and Latent Hepatic Plasmodia Ex Vivo: Potentials and Pitfalls

For a long while, 8-aminoquinoline compounds have been the only therapeutic agents against latent hepatic malaria parasites. These have poor activity against the blood-stage plasmodia causing acute malaria and must be used in conjunction with partner blood schizontocidal agents. We examined the impacts of one such agent, chloroquine, upon the activity of primaquine, an 8-aminoquinoline, against hepatic stages of Plasmodium cynomolgi, Plasmodium yoelii, Plasmodium berghei, and Plasmodium falciparum within several ex vivo systems—primary hepatocytes of Macaca fascicularis, primary human hepatocytes, and stably transformed human hepatocarcinoma cell line HepG2.

For a long while, 8-aminoquinoline compounds have been the only therapeutic agents against latent hepatic malaria parasites. These have poor activity against the blood-stage plasmodia causing acute malaria and must be used in conjunction with partner blood schizontocidal agents. We examined the impacts of one such agent, chloroquine, upon the activity of primaquine, an 8-aminoquinoline, against hepatic stages of Plasmodium cynomolgi, Plasmodium yoelii, Plasmodium berghei, and Plasmodium falciparum within several ex vivo systems—primary hepatocytes of Macaca fascicularis, primary human hepatocytes, and stably transformed human hepatocarcinoma cell line HepG2. Primaquine exposures to formed hepatic schizonts and hypnozoites of P. cynomolgi in primary simian hepatocytes exhibited similar 50% inhibitory concentration (IC₅₀) values near 0.4 μM, whereas chloroquine in the same system exhibited no inhibitory activities. Combining chloroquine and primaquine in this system decreased the observed primaquine IC₅₀ for all parasite forms in a chloroquine dose-dependent manner by an average of 18-fold. Chloroquine also decreased the primaquine IC₅₀ against hepatic P. falciparum in primary human hepatocytes, P. berghei in simian primary hepatocytes, and P. yoelii in primary human hepatocytes. Chloroquine had no impact on primaquine IC₅₀ against P. yoelii in HepG2 cells and, likewise, had no impact on the IC₅₀ of atovaquone (hepatic schizontocide) against P. falciparum in human hepatocytes. We describe important sources of variability in the potentiation of primaquine activity by chloroquine in these systems. Chloroquine potentiated primaquine activity against hepatic forms of several plasmodia. We conclude that chloroquine specifically potentiated 8-aminoquinoline activities against active and dormant hepatic-stage plasmodia in normal primary hepatocytes but not in a hepatocarcinoma cell line.

Determination of Cytochrome P450 Isoenzyme 2D6 (CYP2D6) Genotypes and Pharmacogenomic Impact on Primaquine Metabolism in an Active-Duty US Military Population

Background

Plasmodium vivax malaria requires a 2-week course of primaquine (PQ) for radical cure. Evidence suggests that the hepatic isoenzyme cytochrome P450 2D6 (CYP2D6) is the key enzyme required to convert PQ into its active metabolite.

Methods

CYP2D6 genotypes and phenotypes of 550 service personnel were determined, and the pharmacokinetics (PK) of a 30-mg oral dose of PQ was measured in 45 volunteers. Blood and urine samples were collected, with PQ and metabolites were measured using ultraperformance liquid chromatography with mass spectrometry.

Results

Seventy-six CYP2D6 genotypes were characterized for 530 service personnel. Of the 515 personnel for whom a single phenotype was predicted, 58% had a normal metabolizer (NM) phenotype, 35% had an intermediate metabolizer (IM) phenotype, 5% had a poor metabolizer (PM) phenotype, and 2% had an ultrametabolizer phenotype. The median PQ area under the concentration time curve from 0 to ∞ was lower for the NM phenotype as compared to the IM or PM phenotypes. The novel 5,6-ortho-quinone was detected in urine but not plasma from all personnel with the NM phenotype.

Conclusion

The plasma PK profile suggests PQ metabolism is decreased in personnel with the IM or PM phenotypes as compared to those with the NM phenotype. The finding of 5,6-ortho-quinone, the stable surrogate for the unstable 5-hydroxyprimaquine metabolite, almost exclusively in personnel with the NM phenotype, compared with sporadic or no production in those with the IM or PM phenotypes, provides further evidence for the role of CYP2D6 in radical cure.

Clinical Trials Registration

NCT02960568.

8-Aminoquinoline Therapy for Latent Malaria

The technical genesis and practice of 8-aminoquinoline therapy of latent malaria offer singular scientific, clinical, and public health insights. The 8-aminoquinolines brought revolutionary scientific discoveries, dogmatic practices, benign neglect, and, finally, enduring promise against endemic malaria. The clinical use of plasmochin-the first rationally synthesized blood schizontocide and the first gametocytocide, tissue schizontocide, and hypnozoitocide of any kind-commenced in 1926. Plasmochin became known to sometimes provoke fatal hemolytic crises. World War II delivered a newer 8-aminoquinoline, primaquine, and the discovery of glucose-6-phosphate dehydrogenase (G6PD) deficiency as the basis of its hemolytic toxicity came in 1956. Primaquine nonetheless became the sole therapeutic option against latent malaria. After 40 years of fitful development, in 2018 the U.S. Food and Drug Administration registered the 8-aminoquinoline called tafenoquine for the prevention of all malarias and the treatment of those that relapse. Tafenoquine also cannot be used in G6PD-unknown or -deficient patients. The hemolytic toxicity of the 8-aminoquinolines impedes their great potential, but this problem has not been a research priority. This review explores the complex technical dimensions of the history of 8-aminoquinolines. The therapeutic principles thus examined may be leveraged in improved practice and in understanding the bright prospect of discovery of newer drugs that cannot harm G6PD-deficient patients.